County profile · official public datasets

Plymouth County, IA

432 public highway bridges tracked in the FHWA National Bridge Inventory: 52.1% rated Good, 26.4% Fair, 21.5% Poor.

432
Public bridges: 52.1%
Good condition: 21.5%
Poor condition

The verdict

According to the Federal Highway Administration's 2024 National Bridge Inventory.

Plymouth County pairs a higher-than-median Poor-condition share among rated counties in the state with an older average build year than the state inventory.

60th
of 99 on Poor share: 21.5%
Poor-condition share: 432
public highway bridges: 1974
avg year built

Poor is an FHWA condition category from the lowest applicable component rating (4 or below), not a live crossing-safety or closure decision. Rankings cover counties with at least five public highway bridges.

Plymouth County Bridge Snapshot

According to the Federal Highway Administration's 2024 National Bridge Inventory, Plymouth County ranks #60 of 99 Iowa counties with at least 5 public highway bridges on Poor-condition share (lower share ranks better; currently 21.5%). Plymouth County, Iowa contains 432 public highway bridges. Roughly 52.1% are rated “Good,” 26.4% “Fair,” and 21.5% “Poor.” Poor means the lowest applicable deck, superstructure, substructure, or culvert rating is 4 or below on the 0–9 NBI scale. It is a condition category, not a live safety or closure decision.

Responsible agencies inspect covered bridges on risk-based intervals under current NBIS rules and submit inventory data to FHWA annually. The list below shows every NBI-tracked bridge in the county. Use the “Sort by” controls to change the ordering, and click any row for its reported inspection date, traffic count, and condition fields.

Source: FHWA National Bridge Inventory. See methodology for full definitions.

Total bridges

432

Public highway bridges in Plymouth County

Good condition

52.1%

225 of 432 rated 7-9

Fair condition

26.4%

114 of 432 rated 5-6

Poor condition

21.5%

93 of 432 rated 4 or below

In-state Poor-share rank

#60 of 99

Among Iowa counties with 5+ bridges, lower share ranks better

Condition distribution

Share of 432 NBI-tracked bridges by lowest applicable component rating.

Good

52.1% (225)

Fair

26.4% (114)

Poor

21.5% (93)

Plymouth County's Poor-condition bridges carry 3,697 vehicle crossings a day

That is 0.8% of the county's reported daily bridge traffic on 21.5% of its bridges: Poor-condition structures carry less traffic than their share of the inventory. Each Poor bridge averages 40 crossings a day against 938 for a Good-condition one. A Poor rating is a condition category, not a closure or a live safety judgement; it means the lowest applicable component rating is 4 or below.

Computed over the 432 of 432 bridges in this county that report an average daily traffic count. Source: FHWA National Bridge Inventory, 2024 release.

46.1% of Plymouth County's bridges have passed the 50-year mark

199 of the 432 bridges with a recorded construction year were built in 1974 or earlier. Age alone is not a defect: a well-maintained older bridge can rate Good, and 52.1% of this county's inventory does. Even so, this older cohort is the population from which future Poor ratings are drawn, which is why the age profile below matters alongside the condition split above.

Cutoff derives from the 2024 release year minus a 50-year design life. Source: FHWA National Bridge Inventory, 2024 release.

One structure carries 17% of the county's Poor-condition bridge traffic

FM over STEPHENS CREEK, built 1960, carries 628 vehicles a day, the most of any Poor-rated structure here. The county's Poor-condition exposure is concentrated rather than evenly spread, so the 93-bridge count understates how much of the daily burden sits on a single crossing.

Share is of the 3,697 daily crossings on Poor-rated bridges that report a traffic count. Source: FHWA National Bridge Inventory, 2024 release.

1900s

41
1910s

2
1920s

3
1930s

6
1940s

35
1950s

43
1960s

46
1970s

54
1980s

27
1990s

48
2000s

67
2010s

47
2020s

13

What this shows Construction peaked in the 2000s, when 67 of the county's bridges still in the inventory were built, which is 15.5% of those with a recorded year. That cohort is now 15 to 24 years old in the 2024 inventory. Its average build year is 1974, 5 years older than the Iowa average of 1979.

How Plymouth County compares with Iowa's closest counties by inventory size

The 5 Iowa counties nearest Plymouth County in number of bridges, with their condition splits. Its 21.5% Poor share is the 4th-lowest of these 6. Its average build year of 1974 against a peer-group mean of 1982 is 8 years older.

Showing the 5 closest of 98 Iowa counties eligible for comparison.

Bridge inventory size and condition shares for Plymouth County and the 5 Iowa counties closest to it in inventory size
County	Bridges	Good	Fair	Poor	Avg. built
Black Hawk County	489	52.8%	40.7%	6.5%	1983
Woodbury County	455	39.3%	39.6%	21.1%	1985
Sioux County	449	62.4%	34.7%	2.9%	1982
Plymouth County (this page)	432	52.1%	26.4%	21.5%	1974
Jasper County	382	38.5%	32.5%	29.1%	1977
Benton County	374	34.2%	42.8%	23%	1984

Each share is of that county's own total tracked bridges; peers are the counties with 5 or more bridges whose inventory size is closest to Plymouth County's. Source: FHWA National Bridge Inventory, 2024 release.

Nationwide counties with similar inventory size or average build year

Two NBI-derived peer sets for Plymouth County, both outside Iowa so the neighborhoods are not the same-state size table above.

Similar inventory size

Nearest county inventories outside IA (432 bridges here).

Platte County, NE · 433 bridges
Stoddard County, MO · 434 bridges
Pinellas County, FL · 435 bridges
Tioga County, PA · 429 bridges

Similar average build year

Nearest mean year-built outside IA (1974 here).

Knox County, TN · avg 1974 · 588 bridges
Summit County, OH · avg 1974 · 563 bridges
Onondaga County, NY · avg 1974 · 472 bridges
Orange County, NY · avg 1974 · 472 bridges

Source: FHWA National Bridge Inventory, 2024 release. Peers floor at 10 bridges so stub inventories cannot own the match.

All 432 Good 225 Fair 114 Poor 93

Good-condition bridges (225)

Sort:

Plymouth County Bridge Snapshot

Condition distribution

Plymouth County's Poor-condition bridges carry 3,697 vehicle crossings a day

46.1% of Plymouth County's bridges have passed the 50-year mark

One structure carries 17% of the county's Poor-condition bridge traffic

How Plymouth County compares with Iowa's closest counties by inventory size

Nationwide counties with similar inventory size or average build year

Similar inventory size

Similar average build year

Good-condition bridges (225)

200TH ST

OTTER AVE

MARBLE AVE

NB US 75

330TH ST

FM

QUARRY ROAD

IMPERIAL RD

IA 140

HEDGE AVE

240TH ST

130TH ST

FM C-70

IRIS AVE

IA 60 SB

FM

OTTER AVE

280TH ST

270TH ST

FM C-70

US 75 NB

US 75 SB

320TH ST

FM

190TH ST

FM

220TH ST

FM

160TH ST

IRIS AVE

310TH ST

IA 3

OTTER AVE

150TH ST

IDEAL ROAD

FM

NATURE AVE

DIAMOND AVE

C 38

120TH ST

300TH ST

310TH ST

FAWN AVE

120TH ST

C12 110TH ST

FM

230TH ST

FM K-42

TAMARACK AVE

FAWN AVE

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